The present invention relates to photovoltaic systems for providing electricity from solar power and in particular to a junction box for electrically combining banks of photovoltaic cells.
Sunlight, provides a promising source of energy that avoids many of the problems of burning fossil fuels. While sunlight can be readily concentrated and used for heating purposes, an increasingly attractive option is the direct generation of electrical power from sunlight using photovoltaic cells. Photovoltaic cells may be fabricated of semiconductor materials and take advantage of the photovoltaic effect to convert light into electrical voltage.
The voltage provided by each photovoltaic cell is relatively small (approximately 0.5 volts for standard cell) and accordingly they are normally combined in series to provide a desired working voltage. This series combination of photovoltaic cells may be managed by a junction box which includes bypass diodes shunting individual photovoltaic cells or sets of series connected cells. The current produced by series connected cells is limited by the lowest current cell, for example, one that is in shade, and in this case, the diodes provide a bypass around the shaded cells and prevent current from being driven through the shaded cell such as can cause heating of the cell. The diodes may also bypass cells that have failed.
Power from the junction boxes may be combined and provided to an inverter which converts the direct current of the photovoltaic cells into alternating current for connection to the grid or local electrical system.
The effectiveness and safety of a photovoltaic system is strongly dependent on the proper operation of each of the photovoltaic cells; however, when the cells are combined and during normal variation in cell output their operating state is difficult to establish. This can lead to unexpected or catastrophic cell failure.